It took 7 million years for us to become us, and now there are 7 billion of us on the planet.

"The Human Odyssey," an exhibit that opens Friday at the California Academy of Sciences, brings the latest scientific evidence of the long and many-branched steps that humanity has taken to reach today's stage in evolution.

"We're telling a remarkable human story," said Zeresenay Alemseged, the academy's Ethiopian-born curator of anthropology. "It tells about the earliest prehumans we know of, about the first to walk on two legs, the first to make tools, and the emergence of the big brains we have today."

In the exhibit's small space - fittingly, it occupies part of the popular African Hall near an upright gorilla - more than a dozen casts of hominid skulls are mounted for visitors to touch and feel. Startling video animations that visitors can control show changes in faces, in stature, and in the motions of upright walking, from chimpanzees to Lucy to modern humans.

Evolving humans

The skulls show how features evolved over the millennia.

There's a strange creature whose remains were found in the Sahel desert of North Africa. It was named Sahelanthropus, 7.6 million years old, and it may have been a common ancestor of both chimps and humans, some scientists argue.

The procession of mounted skulls continues, each with accompanying details of their forms.

There's Ardipithecus ramidus, known as Ardi, 4.4 million years old, discovered in Ethiopia's Afar desert by UC Berkeley's Tim White and his colleagues.

And of course there's Lucy, Australopithecus afarensis, 3.3 million years old and probably the most famous fossil ever unearthed.

Lucy occupies her own large niche in the exhibit, giving the public an opportunity to ponder a full-scale, mounted replica of her complete skeleton. It stands 3 feet, 7 inches tall and is enclosed in Plexiglas.

Ten other hominid skulls are mounted too, with details of all the changes in their lives.

The fossil record shows that modern humans emerged some 200,000 years ago, and the exhibit's focus displays the routes our ancestors followed out of Africa.

Huge illuminated world maps display the routes the early humans followed over the millennia. Moving lines, arrow-tipped and dated, flow from Africa through the Middle East and into Europe, across Asia, into North America some 13,000 years ago, and down to Australia even earlier, some 45,000 years ago.

Diversity out of Africa

Near the end of the exhibit a brilliant panel of colored lights tells a story that few know. It depicts the broad genetic diversity of Africa's modern humans before they left that continent some 90,000 to 70,000 years ago. There were many variations of the human gene pool at that time, and then most of the strains disappeared during humanity's move "Out of Africa."

Alemseged notes with fascination that there was a "pinch" of populations during that time when scientists believe the human race nearly became extinct.

One theory holds that during those thousands of years, a massive volcanic eruption, more violent than any before or since, caused 10 years of extreme climate changes that destroyed huge numbers of humans.

The estimated human population was reduced to as few as 10,000 reproductive pairs, and their genetic descendants are what populates the world today, Alemseged said.

He is noted among anthropologists for discovering a 3.3 million-year-old fossil child he named Salem, meaning "peace" in the Amharic language. Salem was widely described as "Lucy's Child" although she was obviously many thousands of years older.

"We are describing what makes us all human," Alemseged said. "It's the making of our own human species, and we are in fact all African."